In order to obtain a paper which is suited for printing and other commercial applications different kinds of performance chemicals are added to the cellulosic suspensions in the paper mill. One type of compounds which are added serve the purpose of rendering the paper more resistant to the penetration of liquids such as aqueous solutions and are commonly referred to as sizing agents. These sizing agents often contain hydrophobic moieties, whereby the compounds are either covalently bound to the cellulosic fibres or alternatively only associated to the fibres. The performance of the sizing agents which are usually added to the suspension in from of a dispersion or emulsion is influenced by numerous factors such as the type of pulp in the suspension, i.e. chemical pulp, mechanical pulp, recycled fibres, other performance additives like retention agent, and above all the amount of contaminants present in the suspension. Contaminants present in the suspension which have an effect on the retention of sizing agents are inter alia ionic compounds ranging from salts to polymers having a substantial molecular weight. The existence of charged polymeric compounds like xylanes in the suspension is often indicated by measuring the cationic demand of the suspension. A higher cationic demand signifies a higher concentration of charged polymeric compounds in the suspension. Suspensions with high cationic demands can originate from type of pulp used, i.e. recycled paper, diluting water used, and the degree of re-circulation of the white-water in the paper-mill. A paper-mill where the white-water is extensively re-circulated tend to end up with a suspension having a high cationic demand. Accordingly, one objective with the present invention is to provide a process for sizing paper, where the paper is formed from a suspension containing cellulosic fibres having a high cationic demand and/or a high conductivity. A further objective is to improve the retention of the sizing agent, specifically the so called first pass retention. The sizing composition according to the present invention may even render paper which is sufficiently sized without the additional addition of retention agents. Further objectives will be apparent hereinafter.
The sizing agent per se is a compound with a hydrophobic character, therefore, it is virtually impossible to disperse/emulsify the sizing agent in an aqueous solution without the presence of compounds which effect or facilitate the formation of a dispersion. In addition, further compounds may be required in order to stabilise the dispersion. The formation of a sizing dispersion prior to the addition of the sizing agent to the suspension is in principle a prerequisite in order to obtain a paper which is sufficiently sized. Normally, the particle size or the weight average particle size of the particles containing the sizing agent has an influence on the sizing efficiency. Better sizing according to common practice is achieved if the particles containing the sizing agent are as small as possible, suitable below 1 μm. However, the object with the present invention is to render a composition comprising aggregates containing the sizing agent which are substantially larger than sizing particles in conventional dispersions. It has surprisingly been shown that the process and composition according to the present invention renders a paper which is sufficiently sized, i.e. a Cobb-value below 30, even without the use of retention agents. In addition, the process and composition significantly improves sizing when applied to cellulosic suspensions having a high cationic demand and/or high conductivity.
In WO 00/34583 a sizing dispersion is disclosed which is stabilised by a cationic colloidal coacervate stabilising agent, where the coacervate agent contains an anionic component and a cationic component. The anionic and cationic components must be present in a proportion so that the zeta potential of the dispersion is at least 20 mV, though a zeta potential of at least 40 mV is more preferable. The objective with the coacervate agent is to stabilise the emulsified or dispersed sizing agent.
U.S. Pat. No. 6,159,339 refers to a paper size consisting essentially of ASA/AKD and degraded, liquid cationic starch.
WO 9833979 relates to an aqueous dispersion containing a cellulose-reactive sizing agent comprising a low molecular weight cationic organic compound having a molecular weight less than 10,000 and an anionic stabilizer.